Accuracy counts – metrology for surface engineering

Dr Mark Gee and Dr Nigel Jennett, both at the National Physical Laboratory in Teddington, UK, discuss the importance of metrology and standardisation for surface engineering.

Surface engineering has the potential to radically transform the structural and functional performance of surfaces. To achieve this, it is essential that designers know how engineered surfaces will perform in a particular application and feel confident to use that knowledge. To fulfil this increasingly pressing industrial need, there are three underlying
requirements:
• To know the actual conditions that exist in an application such as the forces, strains and complex environments imposed upon a component.
• To use or to develop test methods that can recreate these test conditions for samples of materials.
• To derive from the test results useful information and understanding of how the materials behave in a format that can be used in the design of components and to specify the surface engineering solution that will be used.

This focused approach brings additional benefits. It should reveal the key attributes of the engineered surface required for the integrity and durability in operation. These characteristics can then be the focus of quality control processes during production and variability in these attributes can be reduced by careful process design.

To the test

There is an important trade-off in test design, however. Normally, the choice is between a complex test that simulates actual service conditions and gives results that are immediately applicable but are difficult to interpret more generally, and a simple but constrained test that does not mimic service conditions exactly but gives robust measurements of sensitivities to individual parameters. In the latter case, a major requirement in the test design is to know which test conditions can be relaxed and what the effect of this relaxation will be on the results of the tests and their prediction of eventual performance in the application.

In the alternative close-simulation approach, it is important to ensure that the laboratory test replicates the application as closely as possible so that the data produced, with respect to the end use, is robust. Field-testing of prototype components is normally excluded for cost reasons, but in some cases can be the most cost effective approach, particularly where there is complex interaction between many diverse parameters.

Whether taking the simulation or the constrained approach, it is essential to assure that a test is relevant to a particular application. Laboratory analysis using microscopy and other techniques is helpful to identify the mechanisms that are taking place. If the mechanisms are the same in the test and application, then there is greater confidence that the test is valid. The images left, below, show the worn surface from a slipper blade made from WC/Co hardmetal used in the production of concrete tiles. The surfaces of these blades are subjected to abrasive wear from the aggregate in the concrete mix.

A wear test was devised, by the National Physical Laboratory (NPL), using a standard laboratory method to give similar conditions to those in tile processing equipment. The worn surface of the sample subjected to the laboratory test had an identical mode of damage to the slipper blade. This correlation was confirmed by field trials, which showed that the wear predicted by the laboratory tests was consistent with that seen in production.

There are many recognised test methods applicable to such investigations. These include – measurements of thickness, adhesion, hardness, cohesive strength, elastic properties, residual stress, wear and friction performance and microstructural assessment. A summary is given in the table, right.

There are, likewise, many institutes and laboratories that can provide test and measurement expertise, ranging from university laboratories through commercial test houses to internal industrial laboratories. The NPL plays a major role in developing and validating measurement methods for materials properties, including those addressed by surface engineering, and carries out this work in collaboration with industry.

Raise the standard

Once measurement methods have been developed and shown to provide useful information for industry, it is appropriate that they are standardised by national and international standards bodies. This achieves an international consensus on the best procedure through the interaction of recognised experts. In the case of CEN or ISO standards, national standards bodies appoint the experts. For British standards, trade associations and other stakeholders appoint the experts. Standardisation ensures that the appropriate procedures for carrying out measurements are published and available to all.

The result is that best practice in measurement methods is followed, reducing the uncertainty in measurements and improving repeatability within and between laboratories, so that there is confidence in the use of the measurements and the transferability of results. Thus a level playing field is offered, removing barriers to trade and enabling fair comparison of products.

There are several committees within the international and national standards bodies that are concerned with the development of standards for surface engineering. Some deal with coatings and surface treatments directly. Other committees deal with industrial sectors in which surface engineering of specific components is important. An example of the latter is ISO TC 150 ‘Medical Implants’, where there are several important standards covering how the surfaces used in joint prosthetics are prepared and tested.

The main standards committees relevant to surface engineering are given in the table. Some are classified by processing route, as for thermally sprayed coatings, others by material type such as paints and varnishes, metallic coatings, and ceramic coatings. A brief search carried out by NPL on behalf of the IOM3 Surface Engineering Division, (using the British Standards [BS] online search engine which covers ISO and CEN as well as BS), found over 3,000 standards using the search term ‘surface engineering’. They were found in categories as diverse as metallic and other organic coatings, optical and photonic, fine ceramics, paints and varnishes, aerospace, steel wire and other wire products.

It should also be emphasised that other sector bodies such as defence and the oil industry also issue standards that are often more specific in their requirements than the generic regulations issued by the standards bodies.

There is clearly an issue in identifying available standards for a particular measurement or application. All the standards making bodies have web-based search engines that can be used to find standards and the BSI catalogue gives access to most ISO and CEN standards as well as BS standards. BSI also has a subscription-based online facility (British Standards Online, [BSOL]) that has additional facilities. To a large extent, the difficulty experienced comes down to the expertise of the person seeking the information. They have to be able to frame an effective search term and, as a search normally only gives a list of titles and abstracts, the user has to decide which standards to purchase.

The use of standardised test methods has clear benefits for improving the use of test methods. However, it is up to industry to make the most of the available opportunities.